Gas tube overvoltage protectors are widely used for the protection of equipment from overvoltage conditions which may be caused by lightning, power line contact, and the like.
It is also a widely practiced technique to associate various fail-safe arrangements with such tubes and with other types of protectors, e.g. air gap arrestors, to meet various contingencies. For example, the presence of a sustained overload, as where a power line has come in permanent contact with a protected telephone line, produces a concomitant sustained ionization of the gas tube and the resultant passage of heavy currents through the tube. Such currents will in many cases destroy the overvoltage protector and may also constitute a fire hazard.
One approach to this problem involves the association of the gas tube with serially connected fusible elements such that a sustained overload disconnects both the protected device and the protected circuit from the source experiencing the overvoltage condition. (Other arrangements disconnect only the protected circuit, or only the arrestor.)
Another common approach is to employ fusible elements which fuse in the presence of such overloads and provide either a permanent short circuiting of the arrestor directly, or function to trigger another mechanism, e.g. a spring loaded shorting bar, which provides the short circuit connection (generally, the arrestor electrodes are both shorted and grounded). The presence of the permanent short and ground condition serves to flag attention to the fact that the protector has failed or was in a failure mode, thus signalling the need for its inspection or replacement. Examples of this type of fail-safe protection are found in U.S. Pat. Nos. 3,281,625, 3,340,431 and 3,522,570. Of particular relevance to the invention herein is U.S. Pat. No. 3,896,343 which illustrates the use of fusible rings or sleeves on the casing of the gas tube to provide the fail-safe function.
Protector circuits are subject to other types of contingencies as well, including gas tube failure, e.g. by reason of gas leakage. To deal with this possibility, it is a widely practiced technique to combine the gas tube protector with backup protection so that the equipment remains protected in the event the tube fails (becoming an open circuit; a failure producing a short nullifies the protection of the backup gap). Such backup protection is often a requirement specified by utilities such as telephone companies for protectors associated with their equipment.
The fail-safe and backup arrangements described above are also combined in certain gas tube protector units so that there is both fail-safe fusible type protection as well as backup gap protection. One example of such an arrangement is found in U.S. Pat. No. 3,254,179. It is with this combined protection that the invention herein is concerned.